In order to reduce fuel consumption and air pollution by increasing the combustion efficiency of the internal combustion engine which is a kind of combustion engine, it has been a conventional practice to supply ozone, as an auxiliary gas for a mixture including a fuel and air (an air-fuel mixture), to an intake path of the internal combustion engine. For instance, Japanese Unexamined Patent Publication No. H5-44584 (Paragraph No. 0008 and FIG. 1) discloses an internal combustion engine wherein a corona-discharge ozone generator is disposed in an intake manifold as an intake member. The ozone generated by the ozone generator is used for activating oxygen present in the air-fuel mixture sucked in the intake manifold, thereby increasing the combustion rate of the air-fuel mixture.